Yes if Seiya moves back to RF then anyone in the org can play DH. Shaw, Cassie, Mervis, Alcantara, Triantos, Canario, Ballesteros etc. They could all compete in ST and throughout the season, and maybe have a platoon. Chance they could sign a cheaper guy like JD Martinez too.
I have no idea what they'd do with the money saved on Bellinger though if they aren't interested in Soto or a top SP. They're not big pen spenders.